Iraq Prime Minister Haider al-Abadi announced on Monday that a military offensive has begun to seize back the Iraqi city of Mosul after two years of ISIS control.

"We have been battling ISIS for more than two years. We started fighting Isis in the outskirts of Baghdad, and thank God we are now fighting them in the outskirts of Mosul, and God willing the decisive battle will be soon," the CNN quoted Abadi as saying.

"These forces that are liberating you today, they have one goal in Mosul which is to get rid of Daesh and to secure your dignity. They are there for your sake," he said.

Iraqi forces moved into their final positions on Friday, joining Kurdish peshmerga soldiers before an expected advance from the south. Also on the ground are US, British and French Special Forces, which have been advising the peshmerga and will play a prominent role in calling in airstrikes against ISIS targets inside the city.

Ahmed al-Assadi, a lawmaker and spokesman for the militias, said: "We promise you that it will be a great victory fitting with the greatness of Iraq and its history and its people."

But the fight is expected to last weeks, if not months, and if the battles to wrest Falluja and Ramadi from the grip of ISIS are any indication, Mosul is predicted to be a protracted and difficult affair.

The assault on the northern Iraqi city is the most critical challenge yet to ISIS's two-year-old caliphate, which has shredded state authority in the region's heartland, caused a mass exodus of refugees and attempted a genocide of minorities.

ISIS is thought to have about 6,000 fighters ready to defend Mosul, hidden among an estimated civilian population of approximately 600,000, most of whom are expected to flee as the battle intensifies. Before its occupation by Isis, the city was home to more than two million people.

Militants have banned civilians from leaving the city

While leaving can mean trekking through minefields and the risk of punishment by ISIS, those who stay know they face airstrikes, street battles, a potential siege by the Iraqi security forces and the grim possibility of being used as human shields by ISIS.

The US has recently deployed an additional 600 troops to aid in the city's capture, bringing the total number of US personnel to more than 5,200, according to the Pentagon.

Jeff Davis, a Pentagon spokesman, said: "This is the last big holdout in Iraq for ISIL."

The US said on Sunday night that it was proud to stand with its allies in the offensive to retake Mosul.

In a statement, defense secretary Ash Carter said: "This is a decisive moment in the campaign to deliver ISIL a lasting defeat.

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